Do Now… In your notebook: Write a couple sentences or bullet points on the following: What components does something need to have to be a cell? Think about the origin of life on Earth… What’s your idea of the conditions on Earth when it first formed? What is your understanding about how life on earth began? What steps would have needed to happen for life to arise on Earth? What questions do you have about how this might have happened? Now turn to a partner and discuss for 2 min.

AP Bio: 10.6.11 - History of Life on Earth Homework: Study for tomorrow’s test PS 6 due next Thursday (today is the only day we will discuss it) Today’s Agenda: Period 1: Lecture/Notes: How did life begin? Period 2: Test Review, and History of Life Timelines Goals for Today: Define cells Evaluate the evidence for the origins of life on Earth Explain the RNA world hypothesis, and evaluate the evidence for it Identify 10 key events in the history of life on Earth, and explain their importance

AP Biology Unit 3: Life is Cellular monosaccharides polysaccharides fatty acids phospholipids phosphates LIFE ?? amino acids enzymes & other proteins nucleotides ATP nucleic acids (DNA & RNA)

What is LIFE? Life is made of cells that have a metabolism and can self-replicate (reproduce) What makes a cell? Membrane (phospholipid bilayer) Cytosol (fluid inside cell) – a watery solution of nutrients, enzymes, etc. (cytoplasm includes cytosol + organelles outside nucleus) DNA (hereditary info) Proteins made from the same 20 amino acids – used as enzymes, structural components, etc. Ribosomes (translate genetic info into proteins) All cells contain all these components at some point in their “lives”

How did life begin? How did carbon-based monomers (sugars, amino acids, nucleotides...) form in the first place? How did monomers assemble into polymers without enzymes to help? How did genetic material become self-replicating? How did the first cells form?

Early Earth: HOT and BOMBARDED

Chemistry of Early (Pre-Biotic) Earth Atmosphere of CO2 and N2 Also present: CH4, H2S, H2, NH2 What’s missing? No oxygen gas – and since oxygen often helps break things down (think rust, cell respiration, combustion), the lack of O2 makes the environment more conducive to synthesis (more reducing) Lots of lightning and volcanic activity Warm ocean water For life to form… Spontaneous formation of biochemical building blocks (amino acids, nucleotides, etc.) Assemble monomers into polymers without enzymes Self-replicating genetic material Enclose it all in a membrane

Step 1: Spontaneous monomer synthesis Miller & Urey’s experiment Results: a variety of amino acids oily hydrocarbons some nucleosides (pre-cursors to nucleotides) simple sugars

But… New evidence suggests that the early atmosphere was not as reducing as Miller and Urey’s experiment required Another possibility – CO2 and H2 reacting at alkaline thermal vents

Step 2: Abiotic Polymerization No biological catalysts (no enzymes) Experiments have shown that some clay substrates can act as inorganic catalysts – on the clay-y shores of the ocean or a freshwater pond?

Which came first, DNA or proteins? Step 3: Origin of Self-Replicating Genetic Molecules Which came first, DNA or proteins?

Our Current Genetic Situation: DNA RNA protein catalyst protein catalyst rRNA catalyst protein

the chicken-or-the-egg conundrum DNA requires proteins (enzymes) to replicate and to make RNA Making proteins requires DNA and RNA

Ribozymes and the RNA world

Evidence for the RNA World Ribosomes… Made of rRNA wrapped around proteins Experiments on ribosomes show: de-activating ribosomal proteins have no effect on their catalytic ability De-activating rRNA elimates their catalytic ability Thus: rRNA acts as (the most important) biological catalyst – it is a ribozyme Small self-replicating RNA molecules have been synthesized in the lab – they can copy themselves indefinitely without enzymes, just a continuous supply of nucleotides

Step 4: Package it into a membrane

Phylogenetic Tree for all living things Time ---------------------------------------->present Nodes indicate most recent common ancestor of two branches

AP Bio: 10.13.11 - History of Life on Earth Homework: Finish PS 6 Goals for Today: Identify 10 key events in the history of life on Earth, and explain their importance Explain the how, why, and significance of changes in Earth’s atmosphere over time Explain how prokaryotic structures and functions have allowed bacteria to conquer the world Today’s Agenda: Period 1: Earth History Timeline and Discussion Period 2: Group work on PS 7 - Prokaryotes

Timeline Activity: Top 10 Events in the History of Life on Earth Count off by 5’s to form groups of 3-4 people Each person – bring your notes from PS 6 #1 As a group: Choose top 10 events in life’s history On chart paper: Create a timeline that is roughly to scale – but don’t spend more than 5 minutes figuring out the scale If necessary, you can create a “zoom in” timeline for specific portions of your overall timeline Plot each of your top 10 events on the timeline Beneath each event, explain why it is so important – how did it affect life on earth? If you have time, illustrate your timeline so it looks pretty Be ready to explain/present your timeline to the class

Where did the oxygen come from?? Original atmosphere contained no O2 Atmosphere of CO2 and N2 Also present: CH4, H2S, H2, NH2 2.7 bya: Atmospheric oxygen begins to increase… why? Photosynthetic prokaryotes! Cyanobacteria. CO2 + H2O  glucose + O2 How did this affect other life on earth? Obligate anaerobes die off Evolution of aerobes – use O2 for highly efficient breakdown of glucose to yield high amounts of ATP (aerobic cellular respiration)

The little guys that changed the world…